The present invention relates generally to a control system for automatic transmissions of automatic vehicles and more particularly to a control system for applying minimum hydraulic pressure to the hydraulic servos of frictional engagement means such as front and rear clutches and front and rear bands when an automotive vehicle is traveling at a constant speed and for increasing the hydraulic pressure applied to the hydraulic servos for a time interval required for engaging the clutches and/or applying the brake bands when upshift, downshift or range shift occur. An oil pump, which is a hydraulic pressure source, is controlled by a prime mover such as an electric motor or by any other driving means other than the output shaft of the automatic vehicle.
Control systems for fluid type automatic transmissions generally include an oil pump for applying hydraulic pressure to frictional engagement means, a torque converter and a lubrication system. Generally two oil pumps, i.e. a front oil pump and a rear oil pump, are provided in order to secure a desired hydraulic pressure at low speed and to minimize noise and horsepower loss at high speed. The former is driven by a prime mover whereas the latter is driven by the output shaft of the transmission. Since automatic transmissions with two oil pumps are generally large in size and complex in construction, the recent trend is toward use of only one front oil pump in such a manner that the hydraulic pressure produced by the front oil pump is increased or decreased by a valve mechanism when an automotive vehicle is traveling at a constant speed or is accelerated. Since the rotational speed of the oil pump ranges from 500 to 6,000 rpm, the discharge of the oil pump is less when the prime mover is rotating at a low speed so that the hydraulic pressure and lubrication become insufficient. When upshift, downshift or range shift occur when the hydraulic pressure is insufficient, hydraulic response delay of the hydraulic servos occurs and the hydraulic pressure tends to be applied to only one hydraulic servo so that the hydraulic pressure in other hydraulic servos tends to drop. This results in short service life of the frictional engagement means. On the other hand when the prime mover rotates at a high speed, excess hydraulic pressure is produced thereby causing unstable hydraulic control, vibrations in the valve system of the device, cavitation and similar problems resulting from flow forces. Furthermore, horsepower consumption is increased.
Accordingly, it is a primary object of the present invention to provide a control system for automatic transmissions of automotive vehicles in which a single oil pump is driven by a prime mover or other suitable driving means, except the output shaft of an automatic transmission, in such a manner that the pump discharge may be maintained minimum when the vehicle is traveling at a constant speed, thereby minimizing the horsepower consumption, with the pump discharge being increased during shifting thereby preventing hydraulic response delay and undesired decrease in hydraulic pressure.